The present invention relates to a novel trueing and dressing tool for trueing and dressing a variety of types of grinding wheels and is especially adaptable for use in connection with surface grinding machines. The present invention also relates to a method for trueing and dressing grinding wheels having vitrified-bonded cubic boron nitride (CBN) abrasive using a stationary, fine point trueing and dressing block, mounted in a base in engaging proximity to a traversed, rotating grinding wheel.
A number of grinding wheels are known to those skilled in the art including, for example, conventional aluminum oxide and silicon carbide grinding wheels, resin-bonded and vitrified-bonded CBN grinding wheels, as well as diamond grinding wheels. However, regardless of the type of abrasive employed in the grinding wheel, it is necessary to periodically true and dress the grinding wheel in order to maintain an open and aggressive grinding surface of a known profile. An open and aggressive surface condition on a grinding wheel is generally desirable since an open grinding surface is less likely to burn a workpiece and requires less grinding power than a grinding wheel having closed, or dull surface.
Grinding wheels are applied by wide variety of devices including surface grinding machines. Such a surface grinding device utilizes a wheel mounted on a rotating armature operable in numerous axes to grind a workpiece, typically mounted onto the table of the surface grinder, into a desired shape of a given surface condition. Because a grinding wheel may need to be trued and dressed while a work piece remains mounted on the machine, the space available to mount a trueing and dressing device is limited and becomes a consideration in the type of device chosen.
A variety of methods for trueing and dressing grinding wheels are known in the art; however, each have drawbacks and disadvantages, particularly in regard to trueing and dressing grinding wheels whose abrasive material is diamond or vitrified-bonded CBN which are mounted on surface grinding machines.
One prior art method is disclosed in U.S. Pat. No. 2,791,211 to Nagy and involves periodically indexing a diamond-tip dressing tool in relation to the grinding wheel so that in all indexing positions the diamond is in contact with the wheel in a direction of hard grain, forming an angle of between 30.degree. and 45.degree. to the crystal axis of the diamond. While such a single point diamond tool is effective for dressing conventional grinding wheels made of aluminum oxide or silicon carbide, the diamond tip of such tool is subject to rapid wear and is generally ineffective for use in dressing grinding wheels employing diamond or vitrified-bonded CBN abrasives.
Another prior art method is disclosed in U.S. Pat. No. 4,866,887 to Imai, et al., and involves first trueing the grinding wheel with a nib type trueing tool by making several passes across the grinding wheel at a relatively small infeed rate. In the final traverse feed, after the majority of the crown has been moved from the grinding wheel, the infeed rate of the trueing tool is set at a relatively larger value to form an aggressive cutting edge on the grinding wheel. One disadvantage of this method for trueing and dressing a grinding wheel is the time it takes to complete the number of passes required to true and dress the grinding wheel according to the invention. Perhaps more importantly, however, when such a method is employed to true and dress CBN or diamond grinding wheels, the nib generally suffers from rapid wear and loss of point geometry, thus affecting the trueness of the dressed surface.
A number of alternatives to single point trueing and dressing tools are known in the art and include rotary cup tools and straight wheel tools, using diamond abrasives, such as disclosed in U.S. Pat. No. 4,915,089 to Ruark, et al., which is assigned to the same assignee as the present invention and incorporated by reference into the present disclosure. While such rotary trueing and dressing tools have significantly longer life than single point tools, their wide diamond width is generally ineffective in generating the sharp, aggressive cutting surface on the grinding wheel compared to that produced by single point trueing and dressing tools. Furthermore, rotary trueing and dressing tools may require more wheel head clearance than is available below a grinding wheel in a particular surface grinding application. While the powered rotary dressing tool disclosed in U.S. Pat. No. 4,915,089, overcomes the disadvantage of the wide diamond width by its substitution of a single layer of diamond mounted in an axis perpendicular to the rotational axis of the dressing wheel, the method may still require more space in proximity to the grinding wheel than is available.
Prior art also reveals a large surface, diamond-impregnated, block type trueing and dressing tools designed for stationary use. Such block type devices can mounted, for example, unobtrusively on the table of a surface grinding machine. Periodically, as a grinding wheel needs true and dressed, the wheel can be indexed to the table position where the block is mounted and brought into abrading contact with the block's diamond-impregnated surface. After the trueing and dressing operation is complete, the grinding wheel is indexed into its surface grinding position. Since such block type trueing and dressing tools are stationary, this technology necessarily relies on the positioning controls of the grinding machine, whether manual or programmed, to bring the rotating grinding wheel into abrading contact with the trueing and dressing surface of the block.
While offering the advantages of being smaller, less expensive and simpler to implement than their rotary alternatives, a large surface area diamond-impregnated block, because of its wide band of abrasive, tends to dull and close a grinding wheel and may leave it with a crowned profile, as well. Grinding wheels in such a dull and closed condition are not desirable since they can generate excessive heat during the grinding process, causing the wheel to burn the work piece. Furthermore, a crowned grinding wheel profile generally affords the artisan less control over the profile of the work piece.
While such prior art methods may be employed despite their respective shortcomings, manufacturers are always concerned with improving the efficiency of the trueing and dressing process. Such improvements should include reducing the time required to true and dress a grinding wheel, reducing the costs associated with the trueing and dressing process, and improving the quality of the profile and surface condition of the trued grinding wheel.